1. Field of the Invention
The present invention relates to a method for purifying a cyanate, which is useful for a sealant, laminate, composite material, molding material and adhesive of electronic parts, efficiently and at high purity from a cyanate crude product solution containing an unsubstituted phenol.
2. Description of the Related Art
As a method for producing a cyanate industrially, there is a well known method in which a cyanogen halide and a phenol are reacted in the presence of a tertiary aliphatic amine (A. W. Snow in "Chemistry and Technology of Cyanate Ester Resins", Chap. 2 (Ed. T. Hamerton), Blackie Academic and Professional, Glasgow, 1994, pp. 7 to 57). However, a cyanate obtained in this method can not avoid remaining of unreacted phenols and the like. Further, when a cyanate is stored for a long period of time, it is easily hydrolyzed to give a phenol. If such a phenol contained in the cyanate, there occurs a problem that the phenol functions as a polymerization catalyst of cyanate and makes difficult the control of thermo-setting reaction using the cyanate.
As a method for purifying a cyanate in a crude product solution containing an unsubstituted phenol, there is reported a distillation method. However, some cyanates have sublimation property under reduced pressure or polymerization property catalyzed by the unsubstituted phenol as a catalyst when heated, therefore, distillation is not satisfactory method for safely obtaining a cyanate of high purity in high yield.
Also reported is a method in which purification is conducted by crystallizing or precipitating a crude product solution of a cyanate. However, a conventionally suggested method in which crystallization is conducted by cooling using only a good solvent gives low yield, and a conventionally suggested method in which crystallization is conducted by contacting a crude product solution of a cyanate with a poor solvent such as hexane, 2-propanol and the like has a problem that the isolation yield is low since removal of a raw material phenol is difficult and further a part of a cyanate is dissolved in the poor solvent.
Moreover, a filtrate obtained in this operation contains alcohol and non-alcoholic solvent, and disposal thereof is complicated, causing obstruction in efficiency of production of a cyanate itself in practice.